An apparatus which illuminates a specimen placed in a transmission electron microscope with the electron beam to observe the shape or structure of the specimen uses an autofocus apparatus that creates a clear projection image of the specimen on a screen from the transmitted electron beam. Various methods have been proposed to focus the electron beam. One of them is disclosed in U.S. Pat. No. 4,680,469 and has been obtained by expanding the image wobbler method in which the excitation current fed to the objective lens is varied while causing the inclined electron beam to hit one point on a specimen surface from two different directions alternately. When the image created at this one point on the screen is not blurred, it follows that the point is the focal point. The prior art autofocus method utilizing the image wobbler method is next described in greater detail.
Referring to FIG. 4, there is shown the prior art autofocus apparatus utilizing the image wobbler method. FIG. 5 shows waveforms produced at various locations in the apparatus shown in FIG. 4, for illustrating the operation. FIG. 6 is a diagram illustrating how the focal point is found. The autofocus apparatus comprises deflection coils 1 for deflecting the electron beam, an objective lens 3, an imaging lens 4, a screen 5, a control unit 11, a driver circuit 12 for exciting the deflection coils 1, a D/A converter circuit 13, a current stabilizer circuit 14, a memory 15, a synchronizing signal-generating circuit 16, and a TV camera 17. A specimen is indicated by 2.
In the operation of the system shown in FIG. 4, the electron beam hits a point P on the specimen 2. The beam is inclined at an angle .theta. from the optical axis 0 by the deflection coils 1 as indicated by the solid line 6 or the broken line 7. The solid line 6 and the broken line 7 are symmetrical with respect to the optical axis 0. When the electron beam is inclined as indicated by the solid line 6 or the broken line 7, the beam is referred to herein as being inclined forwardly (+) or rearwardly (-), respectively. The specimen 2 is placed between the pole pieces (not shown) of the objective lens 3. The imaging lens 4 which is shown in the form of a block consists of three or four stages of intermediate and projector lenses. The combination of the excitation currents fed to the lenses from a lens power supply 19 is variably set by a magnification-setting circuit 18. A transmission electron micrograph of the specimen 2 is focused onto the screen 5 consisting of a fluorescent screen at a desired magnification. The solid line 8 indicates a magnified image when the electron beam is inclined forwardly (+). The broken line 9 indicates a magnified image when the beam is inclined rearwardly (-).
The control unit 11 consists of a microprocessor or the like and performs processing for automatically focusing the electron beam. In particular, the control unit 11 instructs the deflection coil driver circuit 12 to incline the beam forwardly (+) or rearwardly (-). The control unit 11 causes the D/A converter circuit 13 to produce an excitation current to the objective lens 3 via the current stabilizer circuit 14. The control unit 11 orders the TV camera 17 to pick up the projection image, and performs arithmetic operations to find the focal point from the obtained projection image.
The driver circuit 12 is instructed to produce a given excitation current by the control unit 11, for inclining the electron beam forwardly (+) or rearwardly (-), the current being fed to the deflection coils 1. The D/A converter circuit 13 converts the digital excitation current sent from the control unit 11 into analog form and supplies the analog excitation current to the current stabilizer circuit 14. Then, the stabilizer circuit 14 creates the excitation current specified by the control unit 11 and supplies the current to the objective lens 3.
The memory 15 comprises a backing store, a RAM, and a ROM. The backing store hold files made up of data about the projection image picked up by the TV camera 17. The RAM acts as the work area in which processed data is held. Various kinds of data necessary for performing a series of autofocus operations are stored in the ROM. The synchronizing signal-generating circuit 16 produces a synchronizing signal supplied to the TV camera 17, as well as clock pulses necessary for the operation of the control unit 11. The camera 17 picks up the image of the specimen 2 projected on the screen 5 from the rear side of the screen under instructions from the control unit 11. The data about the image picked up constitutes the files held on the backing store of the memory 15.
When an appropriate operation means such as an autofocus start switch (not shown) is operated to start a series of autofocus operations, the control unit 11 produces a digital signal of a level indicated by 20 in FIG. 5(b) at an instant t.sub.0 to supply a first given excitation current to the objective lens 3. The digital signal is converted into analog form by the D/A converter circuit 13, and then the current stabilizer circuit 14 furnishes an electrical current corresponding to the level 20 (FIG. 5(b)). As a result, the magnetic field set up by the objective lens 3 varies as indicated by 21 in FIG. 5(a). After a lapse of a given time T.sub.0 from the instant t.sub.0, or at an instant t.sub.1, the control unit 1 sends a control signal 22 (FIG. 5(c)) to the driver circuit 12, which then produces an excitation current to the deflection coils 1 to incline the electron beam forwardly (+), for example, according to the control signal 22. Thus, the beam is inclined at an angle of .theta. by the magnetic field 23 (FIG. 5(d)) produced by the deflection coils 1. After the magnetic fields generated by the objective lens 3 and the coils 1 have stabilized and the tilt of the beam has stabilized, the control unit 11 sends a control signal 24 (FIG. 5(e)) to the TV camera 17 to pick up the projection image on the screen 5 during the given period T.sub.1 from an instant t.sub.2 to an instant t.sub.3. The collected data is stored in the memory 15. When the collection of data about the image created by the forwardly inclined beam is completed, the control unit supplies a control signal 25 (FIG. 5(c)) to the driver circuit 12 at an instant t.sub.4, so that the beam is inclined rearwardly (-) by the magnetic field 26 (FIG. 5(d)) produced by the deflecting coils 1. After the magnetic field generated by the coils and the tilt of the beam have stabilized, the control unit 11 produces a control signal 27 (FIG. 5(e)) to the TV camera 17 to pick up the projection image on the screen 5 during the given period T.sub.1 from an instant t.sub.5 to an instant t.sub.6. The collected data about the image is stored in the memory 15. When the collection of data about the image created by the rearwardly (-) inclined beam is complete, the control unit 11 calculates the difference between the accepted two images and data about the difference is stored in the memory 15.
The two images can be two-dimensional images. The difference between data on a one-dimensional line may be calculated. Also, the differences between data on unparallel lines may be calculated. It is now assumed that the beam is inclined forwardly (+) by the objective lens 3 excited with a certain current and an image 30 (FIG. 6) is picked up, and that the beam is inclined rearwardly (-) by the objective lens 3 and an image 31 (FIG. 6) is picked up. In this case, the difference between the data on any of unparallel lines 32, 33, 34, and 35 may be calculated. Also, the differences between the data on all the lines 32-35 may be obtained and combined to find the difference between the images picked up when the beam is inclined forwardly (+) and rearwardly (-), respectively.
After the processing at one excitation current level to the objective lens 3 is completed in this way, the control unit 11 delivers a digital signal of a level 28 (FIG. 5(b)) at an instant t.sub.7 to supply a second given excitation current to the objective lens 3. Subsequently, the same processing as the foregoing is conducted. In particular, the electron beam in inclined forwardly (+), and data about the image is collected. Then, the beam is inclined rearwardly (-), and data about the image is collected. In this way, the control unit 11 accepts data about the images created by the electron beam inclined forwardly (+) and rearwardly (-), respectively, while changing the excitation current fed to the objective lens 3 in a stepwise fashion within a given range. The difference between the obtained two images is calculated for each value of the excitation current. After the completion of the collection of the data about the images, a point is found at which the difference between the images is minimal. This point is taken as the focal point. The excitation current under this condition is supplied to the objective lens 3. Consequently, a projection image is focused onto the screen 5.
Unfortunately, when the prior art image wobbler method is used, it takes a long time to find the focal point. More specifically, the objective lens 3 contains a yoke and pole pieces made of a ferromagnetic substance that shows magnetic aftereffects. Therefore, when the objective lens is instructed to produce the excitation current 20 (FIG. 5(b)) by the control unit 11, the lens cannot immediately respond. The magnetic field does not stabilize until a period T.sub.3 (FIG. 5(a)) passes. Although the period T.sub.3 differs according to the kind of magnetic substance, the period is about 0.5 to 1.0 second for the magnetic substance usually used in the objective lens.
The same phenomenon occurs for the deflecting coils 1. Because of the magnetic aftereffects of the ferromagnetic substance forming the deflecting coils 1, it takes long for the magnetic field inclining the electron beam to stabilize. When the amount of the electrical current is increased, a period of T.sub.4 (FIG. 5(d)) is required. When the amount of the electrical current is reduced, a period of T.sub.5 is required. For the normally used deflection coils, the periods T.sub.4 and T.sub.5 are about 0.1 to 0.2 second.
In order to obtain accurate image data, the data must be collected when the magnetic fields produced by the objective lens 3 and the deflection coils 1 are both stabilized. For this reason, when a first set of image data 24 (FIG. 5(e)) is collected, the time interval between the instants t.sub.2 and t.sub.0 must be at least 0.5 second. When a second set of image data is collected, the interval between instants t.sub.5 and t.sub.4 must be at least 0.2 second.
In this way, the autofocus apparatus utilizing the prior art image wobbler method is required to wait long until image data is allowed to be collected whenever the excitation current fed to the objective lens 3 is varied. Accordingly, where it is necessary to switch the excitation current fed to the objective lens 3 between numerous close levels as encountered in the case of autofocus action at high magnifications, a long time is taken to detect the focal point. Hence, the prior art apparatus has not been practical.